This invention relates to projection lens systems for use in projection televisions and, in particular, to projection lens systems having improved correction of higher order off-axis aberrations, especially coma and astigmatism. and thus improved overall modulation transfer functions across the field of view of the lens in comparison to prior lens systems having a similar construction.
U.S. application Ser. No. 08/715,028, now U.S. Pat. No. 5,808,804, the contents of which were published on Mar. 26, 1997 as EPO Patent Publication No. 764,865, discloses projection lens systems for CRT projection televisions which employ five lens elements and a negative lens unit which is associated with the CRT during use of the lens system.
Although the lens systems of U.S. Pat. No. 5,808,804 work successfully for many applications, it has been found that the level of correction of higher order off-axis aberrations, especially coma and astigmatism, achieved by these prior lens systems result in less than desired overall modulation transfer functions (MTFs) across the field of view of the lens.
The present invention addresses this observed property of these prior lenses while maintaining their otherwise desirable characteristics.
In view of the foregoing, it is an object of the present invention to provide improved projection lens systems for use in projection televisions and, in particular, in rear projection televisions.
More particularly, it is an object of the invention to provide projection lens systems capable of covering a semi-field of view of up to about 40xc2x0 at a f-number at infinity on the order of 1.0, while at the same time providing an image quality which is sufficiently high to satisfy the requirements of modern consumer projection TV systems, including the requirements of high definition televisions (HDTV).
It is an additional object of the invention to provide projection lens systems having the foregoing characteristics which are also partially corrected for axial color.
It is a further object of the invention to provide projection lens systems which can be manufactured at a reasonable cost.
To achieve the foregoing and other objects, the invention provides a projection lens system for use in combination with a cathode ray tube comprising in order from the image (screen) side:
(a) a first lens element (L1) which has a positive power, an image side surface (S1), and an object side surface (S2), wherein:
(i) at least one of the image and object side surfaces is aspherical; and
(ii) the object side surface has a best fit spherical surface which is convex to the cathode ray tube, i.e., the object side surface has a best fit spherical radius R2BF which, under the standard convention for assigning radii to optical surfaces, is less than zero (see Table 5);
(b) a second lens element (L2) which has a negative power, at least one aspheric surface, and is composed of a high dispersion material (e.g., styrene);
(c) a third lens element (L3) which has a positive power and provides a substantial portion of the power of the lens system;
(d) a fourth lens element (L4) which has a weak power, at least one aspheric surface, and an overall meniscus shape which is concave to the cathode ray tube;
(e) a fifth lens element (L5) which has a positive power and at least one aspheric surface; and
(f) a lens unit (UN) which has a strong negative power, is associated with the CRT during use of the lens system, and provides most of the correction for the field curvature of the lens system.
In certain preferred embodiments, the first element""s image side surface (S1) has a clear aperture CA1 which satisfies the following relationship:
xc2xdCA1 greater than Y1xe2x80x83xe2x80x83(1)
where Y1, is the height of the axial marginal ray at S1. By means of this limitation, more effective correction of off-axis aberrations can be achieved using the portion of the first lens element which lies between Y1 and the clear aperture of the lens. Preferably, the first element""s image side surface satisfies the following relationship:
xc2xdCA1 greater than 1.1xc2x7Y1.xe2x80x83xe2x80x83(2)
In other preferred embodiments, the first element""s image side surface has a best fit radius R1BF which satisfies the following relationship:
|R1BF| greater than |R2BF|.xe2x80x83xe2x80x83(3)
That is, the best fit power of the CRT side of the first lens element is greater than the best fit power of the screen side of that element.
In still further preferred embodiments, the lens system has some or all of the following characteristics: (1) each of the first, second, fourth, and fifth lens elements has two aspheric surfaces; (2) the third lens element is biconvex and provides the majority of the power of the lens system, i.e., the focal length of the third lens element is between about 0.8 and about 1.6 times the focal length of the entire lens system; and (3) the fourth lens element has a positive power.
As used herein, the term xe2x80x9cweakxe2x80x9d is used to describe an element whose focal length has a magnitude which is at least about 2.5 times the effective focal length of the entire lens system, and the term xe2x80x9cstrongxe2x80x9d is used to describe an element or unit whose focal length has a magnitude which is less than about 2.5 times the effective focal length of the entire lens system.